Through air drying is well known in the papermaking art. Through air drying has been advantageously accomplished with commercial success using belts having two components, a foraminous element and a resinous framework. Such belts are disclosed in commonly assigned U.S. Pat. No.: 4,514,345, issued Apr. 30, 1985 to Johnson et al.; U.S. Pat. No. 4,528,239, issued Jul. 9, 1985 to Trokhan; U.S. Pat. No. 5,098,522, issued Mar. 24, 1992; U.S. Pat. No. 5,260,171, issued Nov. 9, 1993 to Smurkoski et al.; U.S. Pat. No. 5,275,700, issued Jan. 4, 1994 to Trokhan; U.S. Pat. No. 5,328,565, issued Jul. 12, 1994 to Rasch et al.; U.S. Pat. No. 5,334,289, issued Aug. 2, 1994 to Trokhan et al.; U.S. Pat. No. 5,431,786, issued Jul. 11, 1995 to Rasch et al.; U.S. Pat. No. 5,496,624, issued Mar. 5, 1996 to Stelljes, Jr. et al.; U.S. Pat. No. 5,500,277, issued Mar. 19, 1996 to Trokhan et al., U.S. Pat. No. 5,514,523, issued May 7, 1996 to Trokhan et al.; U.S. Pat. No. 5,554,467, issued Sep. 10, 1996, to Trokhan et al.; U.S. Pat. No. 5,566,724, issued Oct. 22, 1996 to Trokhan et al.; U.S. Pat. No. 5,624,790, issued Apr. 29, 1997 to Trokhan et al.; U.S. Pat. No. 5,628,876, issued May 13, 1997 to Ayers et al., and U.S. Pat. No. 5,679,222 issued Oct. 21, 1997 to Rasch et al. all of which patents are incorporated herein by reference.
Papermaking using two belts is also known in the art. Each belt may have a foraminous element and a patterned framework. One belt is used for the forming wire and one is used for the drying belt. The art also teaches discrete pattern elements in the framework, particularly for the forming wire. Suitable examples are found in commonly assigned U.S. Pat. No. 5,503,715 issued Apr. 2, 1996 to Trokhan et al. and U.S. Pat. No. 5,614,061 issued Mar. 25, 1997 to Phan et al., which patents are incorporated herein by reference.
One feature common to each of these patents is the presence of the patterned framework at or above the plane of the foraminous element. The foraminous element serves as a reinforcing structure for the resinous framework. The patterned framework provides deflection conduits for embryonic webs during the papermaking process, and imprints a high density pattern onto the paper formed thereby. The deflection conduits may be discrete, semicontinuous or continuous.
Recently, attempts have been made to design through air drying belts which do not rely upon a resinous framework to provide deflection conduits or imprint the paper. For example, PCT Application WO 95/27821, published Oct. 19, 1995 in the name of Chiu et al., and European Patent Application 0 677 612 A2, published Oct. 18, 1995 in the names of Wendt et al. disclose through drying fabrics which allegedly impart significantly increased cross direction extensibility to the resulting product, while at the same time allegedly also delivering high bulk. Such fabrics may have a top layer dominated by long warp knuckles, and no top surface shute knuckles. Many of these long warp knuckles have no supporting shutes underneath for resistance to compression which occurs during papermaking, particularly during imprinting. The disclosures of the Wendt and Chiu applications are incorporated herein by reference for the limited purpose of showing how to make papermaking fabrics having a jacquard-like weave pattern. Papermaking fabrics woven on a dobby loom or a multiple harness loom may also be suitable for use with the claimed invention. Likewise, clothing woven on more typical looms may also be suitable for use with the claimed invention.
One problem which appears inherent to this approach, or any approach having warps (or shutes) with long unsupported spans, is deflection of the warps (or shutes) during the papermaking process. Such deflection may prevent the fabric from fully imprinting the paper. Collapse of the clothing will have a deleterious effect on the embryonic web, likely reducing its bulk. Imprinting increases fiber-to-fiber bonding and is typically accomplished in conjunction with a Yankee drying drum, but may also be accomplished with other suitable roll/nip combinations or other rigid surfaces. For example, extended nip presses or shoe presses may be used for imprinting. Imprinting has been commercially successful in improving the softness/strength relationship of tissue. Such commercial success is due to the consumer acceptance of multi-region tissue papers created by the combination of imprinting and the deflection conduits.
Wendt et al. purport to avoid the problem associated with deflection of long spans by not imprinting the paper. To do this, Wendt et al. forego the Yankee drying drum and creping operation. However, this approach is infeasible for the large number of existing papermaking plants which rely upon a Yankee drying drum to complete the drying process. Furthermore, not creping the paper can make it difficult to impart adequate machine direction stretch to the final product. In addition, the absence of imprinted (high density) regions often yields tissue with a poorer softness/strength relationship.
Wendt et al. purport to overcome the problems associated with the absence of creping by allegedly making an uncreped through air dried microcontracted sheet. Microcontraction is known from commonly assigned U.S. Pat. No. 4,440,597, issued Apr. 3, 1984 to Wells et al., which patent is incorporated herein by reference. Microcontraction occurs when the paper is transferred from a faster moving first fabric, such as a forming wire, to a slower moving second fabric, such as a through drying fabric or a transfer fabric. However, microcontraction inherently slows the papermaking process to the limiting speed of the slower moving fabric. Slowing the papermaking process increases the cost of manufacture and limits the papermaking capacity of the machinery. Microcontraction of the paper also tends to deteriorate softness and tensile strength.
A different problem known in the art and associated with through air drying belts is sleaziness. Sleaziness refers to movement of warp and shute yarns within the plane of the belt relative to each other. Sleaziness most often occurs with high shed count fabrics. This problem is exacerbated in those fabrics having long floats, as illustrated in the aforementioned Wendt et al. application. Furthermore, fabrics having low density (high projected open area or high void volume) weaves are also prone to sleaziness problems.
Sleaziness can also occur with multi-layer fabrics. Multi-layer fabrics include dual layer fabrics and triple layer fabrics. Dual layer fabrics have stacked warps tied together by the shutes or vice versa. A triple layer fabric has independent first and second layers juxtaposed in face-to-face relationship and held in place by tie yarns. In a multi-layer fabric sleaziness can occur with movement of one layer of the fabric relative to the other layer of the fabric and/or with movement of warps and shutes relative to other warps and shutes within the same layer.
Yet another problem known in the art and associated with papermaking belts is seam strength. Fabrics made on conventional looms are seamed to provide an endless belt suitable for use in papermaking. Resistance to machine direction failure of the seam in tension is known as seam strength. Seam strength is particularly critical in the aforementioned fabrics having high shed counts, low density or long floats.
Accordingly, in one aspect, this invention may provide a papermaking belt having knuckles adequately supported for imprinting of the paper. Furthermore, in one aspect, this invention may provide a papermaking belt having either long warp knuckles, long shute knuckles, or both, which are adequately supported. In one aspect, this invention may further provide such a belt which can be used with a Yankee drying drum or other means for imprinting the tissue. Further, in one aspect, this invention may provide for both single layer and multi-layer papermaking belts having mitigated sleaziness for a particular weave.